Method of Improving Print Performance in Flexographic Printing Plates

ABSTRACT

A method of tailoring the shape of a plurality of relief printing dots created in a photosensitive printing blank during a platemaking process is provided. The photocurable layer is exposed to actinic radiation using an array of UV LED light assemblies and the use of the array of UV LED light assemblies produces relief printing dots having at least one geometric characteristic selected from the group consisting of a desired planarity of a top surface of the relief printing dots, a desired shoulder angle of the relief printing dots and a desired edge sharpness of the relief printing dots.

FIELD OF THE INVENTION

The present invention relates generally to a method of tailoring theshape of printing dots created during the production of relief imageprinting plates to configure such printing dots for optimal printing onvarious substrates.

BACKGROUND OF THE INVENTION

Flexography is a method of printing that is commonly used forhigh-volume runs. Flexography is employed for printing on a variety ofsubstrates such as paper, paperboard stock, corrugated board, films,foils and laminates. Newspapers and grocery bags are prominent examples.Coarse surfaces and stretch films can be economically printed only bymeans of flexography.

Flexographic printing plates are relief plates with image elementsraised above open areas. Generally, the plate is somewhat soft, andflexible enough to wrap around a printing cylinder, and durable enoughto print over a million copies. Such plates offer a number of advantagesto the printer, based chiefly on their durability and the ease withwhich they can be made. A typical flexographic printing plate asdelivered by its manufacturer is a multilayered article made of inorder, a backing or support layer; one or more unexposed photocurablelayers; optionally a protective layer or slip film; and often, aprotective cover sheet.

The support (or backing) layer lends support to the plate. The supportlayer can be formed from a transparent or opaque material such as paper,cellulose film, plastic, or metal. Preferred materials include sheetsmade from synthetic polymeric materials such as polyesters, polystyrene,polyolefins, polyamides, and the like. One widely used support layer isa flexible film of polyethylene terephthalate.

The photocurable layer(s) can include any of the known photopolymers,monomers, initiators, reactive or non-reactive diluents, fillers, anddyes. As used herein, the term “photocurable” refers to a compositionwhich undergoes polymerization, cross-linking, or any other curing orhardening reaction in response to actinic radiation with the result thatthe unexposed portions of the material can be selectively separated andremoved from the exposed (cured) portions to form a three-dimensionalrelief pattern of cured material. Exemplary photocurable materials aredisclosed in European Patent Application Nos. 0 456 336 A2 and 0 640 878A1 to Goss, et al., British Patent No. 1,366,769, U.S. Pat. No.5,223,375 to Berrier, et al., U.S. Pat. No. 3,867,153 to MacLahan, U.S.Pat. No. 4,264,705 to Allen, U.S. Pat. Nos. 4,323,636, 4,323,637,4,369,246, and 4,423,135 all to Chen, et al., U.S. Pat. No. 3,265,765 toHolden, et al., U.S. Pat. No. 4,320,188 to Heinz, et al., U.S. Pat. No.4,427,759 to Gruetzmacher, et al., U.S. Pat. No. 4,622,088 to Min, andU.S. Pat. No. 5,135,827 to Bohm, et al., the subject matter of each ofwhich is herein incorporated by reference in its entirety. More than onephotocurable layer may also be used.

Photocurable materials generally cross-link (cure) and harden throughradical polymerization in at least some actinic wavelength region. Asused herein, “actinic radiation” is radiation that is capable ofpolymerizing, crosslinking or curing the photocurable layer. Actinicradiation includes, for example, amplified (e.g., laser) andnon-amplified light, particularly in the UV and violet wavelengthregions.

The slip film is a thin layer, which protects the photopolymer from dustand increases its ease of handling. In a conventional (“analog”) platemaking process, the slip film is transparent to UV light, and theprinter peels the cover sheet off the printing plate blank, and places anegative on top of the slip film layer. The plate and negative are thensubjected to flood-exposure by UV light through the negative. The areasexposed to the light cure, or harden, and the unexposed areas areremoved (developed) to create the relief image on the printing plate.

In a “digital” or “direct to plate” process, a laser is guided by animage stored in an electronic data file, and is used to create an insitu negative in a digital (i.e., laser ablatable) masking layer, whichis generally a slip film which has been modified to include a radiationopaque material. Portions of the laser ablatable layer are then ablatedby exposing the masking layer to laser radiation at a selectedwavelength and power of the laser. Examples of laser ablatable layersare disclosed, for example, in U.S. Pat. No. 5,925,500 to Yang, et al.,and U.S. Pat. Nos. 5,262,275 and 6,238,837 to Fan, the subject matter ofeach of which is herein incorporated by reference in its entirety.

Processing steps for forming relief image printing elements typicallyinclude the following:

-   -   1) Image generation, which may be mask ablation for digital        “computer to plate” printing plates or negative production for        conventional analog plates;    -   2) Back exposure to create a floor layer in the photocurable        layer and establish the depth of relief;    -   3) Face exposure through the mask (or negative) to selectively        crosslink and cure portions of the photocurable layer not        covered by the mask, thereby creating the relief image;    -   4) Development to remove unexposed photopolymer by solvent        (including water) or thermal development; and    -   5) If necessary, post exposure and detackification.

Removable coversheets are also typically provided to protect thephotocurable printing element from damage during transport and handling.Prior to processing the printing elements, the coversheet(s) are removedand the photosensitive surface is exposed to actinic radiation in animagewise fashion. Upon imagewise exposure to actinic radiation,polymerization, and hence, insolubilization of the photopolymerizablelayer occurs in the exposed areas. Treatment with a suitable developersolvent (or thermal development) removes the unexposed areas of thephotopolymerizable layer, leaving a printing relief that can be used forflexographic printing.

As used herein “back exposure” refers to a blanket exposure to actinicradiation of the photopolymerizable layer on the side opposite thatwhich does, or ultimately will, bear the relief. This step is typicallyaccomplished through a transparent support layer and is used to create ashallow layer of photocured material, i.e., the “floor,” on the supportside of the photocurable layer. The purpose of the floor is generally tosensitize the photocurable layer and to establish the depth of relief.

Following the brief back exposure step (i.e., brief as compared to theimagewise exposure step which follows), an imagewise exposure isperformed utilizing a digitally-imaged mask or a photographic negativemask, which is in contact with the photocurable layer and through whichactinic radiation is directed.

The type of radiation used is dependent on the type of photoinitiator inthe photopolymerizable layer. The digitally-imaged mask or photographicnegative prevents the material beneath from being exposed to the actinicradiation and hence those areas covered by the mask do not polymerize,while the areas not covered by the mask are exposed to actinic radiationand polymerize. Any conventional sources of actinic radiation can beused for this exposure step. Examples of suitable visible and UV sourcesinclude carbon arcs, mercury-vapor arcs, fluorescent lamps, electronflash units, electron beam units and photographic flood lamps.

After imaging, the photosensitive printing element is developed toremove the unpolymerized portions of the layer of photocurable materialand reveal the crosslinked relief image in the cured photosensitiveprinting element. Typical methods of development include washing withvarious solvents or water, often with a brush. Other possibilities fordevelopment include the use of an air knife or thermal development,which typically uses heat plus a blotting material. The resultingsurface has a relief pattern, which typically comprises a plurality ofdots that reproduces the image to be printed. After the relief image isdeveloped, the resulting relief image printing element may be mounted ona press and printing commenced.

The shape of the dots and the depth of the relief, among other factors,affect the quality of the printed image. In addition, it is verydifficult to print small graphic elements such as fine dots, lines andeven text using flexographic printing plates while maintaining openreverse text and shadows. In the lightest areas of the image (commonlyreferred to as highlights) the density of the image is represented bythe total area of dots in a halftone screen representation of acontinuous tone image. For Amplitude Modulated (AM) screening, thisinvolves shrinking a plurality of halftone dots located on a fixedperiodic grid to a very small size, the density of the highlight beingrepresented by the area of the dots. For Frequency Modulated (FM)screening, the size of the halftone dots is generally maintained at somefixed value, and the number of randomly or pseudo-randomly placed dotsrepresent the density of the image. In both cases, it is necessary toprint very small dot sizes to adequately represent the highlight areas.

Maintaining small dots on flexographic plates can be very difficult dueto the nature of the platemaking process. In digital platemakingprocesses that use a UV-opaque mask layer, the combination of the maskand UV exposure produces relief dots that have a generally conicalshape. The smallest of these dots are prone to being removed duringprocessing, which means no ink is transferred to these areas duringprinting (i.e., the dot is not “held” on plate and/or on press).Alternatively, if the dots survive processing they are susceptible todamage on press. For example small dots often fold over and/or partiallybreak off during printing, causing either excess ink or no ink to betransferred.

As described in U.S. Pat. No. 8,158,331 to Recchia and U.S. Pat. Pub.No. 2011/0079158 to Recchia et al., the subject matter of each of whichis herein incorporated by reference in its entirety, it has been foundthat a particular set of geometric characteristics define a flexo dotshape that yields superior printing performance, including but notlimited to (1) planarity of the dot surface; (2) shoulder angle of thedot; (3) depth of relief between the dots; and (4) sharpness of the edgeat the point where the dot top transitions to the dot shoulder.

The inventors of the present invention have found that the use of one ormore UV LED assemblies in selectively crosslinking and curing sheetphotopolymers can produce a relief image comprising flexo printing dotshaving desirable geometric characteristics.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method oftailoring or modifying the shape of relief printing dots in a reliefimage printing element for optimal printing on various substrates and/orunder various conditions.

It is an object of the present invention to provide an improved methodof producing relief image printing elements comprising dots havingdesirable geometric characteristics.

It is still another object of the present invention to create a reliefimage printing element that comprises printing dots having a superiordot structure in terms of print surface, edge definition, shoulderangle, depth and dot height.

It is another object of the present invention to provide an improvedmethod of creating a relief image printing element having tailoredrelief dots in terms of edge definition, shoulder angle, and/or printsurface.

To that end, in one embodiment, the present invention relates generallyto a method of tailoring the shape of a plurality of relief printingdots created in a photosensitive printing blank during a platemakingprocess, said photosensitive printing blank comprising at least onephotocurable layer disposed on a backing layer, the method comprisingthe steps of:

a) selectively exposing the at least one photocurable layer to a sourceof actinic radiation to selectively crosslink and cure the at least onephotocurable layer; and

b) developing the exposed photosensitive printing blank to reveal therelief image therein, said relief image comprising the plurality ofrelief printing dots;

wherein the source of actinic radiation comprises an array of UV LEDlight assemblies and the use of the array of UV LED light assembliesproduces relief printing dots having at least one geometriccharacteristic selected from the group consisting of a desired planarityof a top surface of the relief printing dots, a desired shoulder angleof the relief printing dots and a desired edge sharpness of the reliefprinting dots.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 depicts a means of characterizing the planarity of a dot'sprinting surface where p is the distance across the dot top, and r_(t)is the radius of curvature across the surface of the dot.

FIG. 2 depicts a flexo dot and its edge, where p is the distance acrossthe dot top. This is used in the characterization of edge sharpnessr_(e):p, where r_(e) is the radius of curvature at the intersection ofthe shoulder and the top of the dot.

FIG. 3 depicts the measurement of the dot shoulder angle θ.

FIG. 4 depicts a UV-Vis plot of Irgacure 651.

FIG. 5 depicts a UV-Vis plot of Darocur TPO.

FIG. 6 depicts a UV-Vis plot of Irgacure 819.

FIG. 7 depicts line speeds of a UV track system for specific inputs.

FIG. 8 depicts a UV trace system expanded slow range.

FIG. 9 depicts the floor build for high level Darocur TPO exposed at 395nm.

FIG. 10 depicts SEM images of various photopolymer compositions exposedusing the Digital Light Labs 365 nm light source at a 25% input level.

FIG. 11 depicts SEM images of various photopolymer compositions exposedusing the UV Process Supply 395 nm light source.

FIG. 12 depicts SEM images of various photopolymer compositions exposedusing the UV Process Supply 415 nm light source.

FIG. 13 depicts SEM images of various photopolymer compositions exposedusing a mixture of 365 nm and 395 nm light sources.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The inventors of the present invention have determined that the shapeand structure of the printing dots in a relief image printing elementhas a profound impact on the way the relief image printing elementprints.

However, the inventors of the present invention have also determinedthat the shape and structure of the printing dots may be altered ortailored using UV LED assemblies. More specifically, the inventors ofthe present invention have determined that it is possible to control thedot angle of the resulting printing dots using UV LED assemblies.

It should be understood that individual UV LED assemblies can beoperated at different wavelengths. It should also be understood that UVlight emitting from a UV LED assembly is over a range of wavelengths,often referred to as the Spectral Energy Distribution, with a peak atone wavelength which is the identified wavelength. For example, UV LEDassemblies are available in wavelengths of 365 nm, 375 nm, 385 nm, 395nm, 405 nm and 415 nm, among others.

In one embodiment, the present invention relates generally to a methodof tailoring the shape of a plurality of relief printing dots created ina photosensitive printing blank during a platemaking process, saidphotosensitive printing blank comprising at least one photocurable layerdisposed on a backing layer, the method comprising the steps of:

a) selectively exposing the at least one photocurable layer to a sourceof actinic radiation to selectively crosslink and cure the at least onephotocurable layer; and

b) developing the exposed photosensitive printing blank to reveal therelief image therein, said relief image comprising the plurality ofrelief printing dots;

wherein the source of actinic radiation comprises an array of UV LEDlight assemblies and the use of the array of UV LED light assembliesproduces relief printing dots having at least one geometriccharacteristic selected from the group consisting of a desired planarityof a top surface of the relief printing dots, a desired shoulder angleof the relief printing dots and a desired edge sharpness of the reliefprinting dots.

The at least one photocurable layer may be coated with a slip film,which is a thin layer, which protects the at least one photocurablelayer from dust and increases its ease of handling. In a conventional(“analog”) plate making process, the slip film is transparent to UVlight, and the printer places a negative on top of the slip film layer.The plate, including the at least one photocurable layer and negativeare then subjected to flood-exposure by UV light through the negative.The areas exposed to the light cure, or harden, and the unexposed areasare removed (developed) to create the relief image on the printingplate. In the alternative, a negative may be placed directly on the atleast one photocurable layer.

In the alternative, in a “digital” or “direct to plate” process, the atleast one photocurable layer is coated with a masking layer, which maybe a slip film that has been modified to include a radiation opaquematerial. In this instance, a laser guided by an image stored in anelectronic data file is used to create an in situ “negative” in themasking layer. Portions of the laser ablatable layer are ablated byexposing the masking layer to laser radiation at a selected wavelengthand power of the laser. Thereafter, the at least one photocurable layerwith the in situ negative thereon, is subjected to flood-exposure by UVlight through the in situ negative. The areas exposed to the light cure,or harden, and the unexposed areas are removed (developed) to create therelief image on the printing plate. Selective exposure to the source ofactinic radiation can be achieved using either the analog or digitalmethod.

The UV LED assemblies can be positioned in a random, mixed manner or insequential rows. For example, in a row of UV LED assemblies, the firstUV LED assembly can emit light at 395 nm, the next UV LED assembly canemit light at 365 nm and the following UV LED assembly can emit light at415 nm, and so on, repeating this pattern throughout the row. The nextrow and subsequent rows can have the same pattern or a differentpattern. Alternatively, all of the UV LED assemblies in a row can emitlight at the same wavelength (i.e., 365 nm, 395 nm, 415 nm), with thenext row having UV LED assemblies that emit light at a differentwavelength, and so on, and the pattern is then repeated for theremaining rows. The pattern or order can also be changed.

In a preferred embodiment, the array of UV LED light assembliescomprises at least four rows. In an array of four rows of bulbs, thefirst and third row could be 365 nm while the second and fourth rowscould be 395 nm. Each wavelength could then be on a separate circuitwhich would have the ability to modulate is intensity separately,allowing the user to customize the dot formation. In one embodiment, thearray of LTV LED lights could be arranged in two alternating rows of 50.

The inventors of the present invention have found that both the angle ofthe UV LED light assemblies and the wavelengths of the UV LED lightassemblies can be varied to produce relief printing dots having thedesired geometric characteristic.

In one preferred embodiment, alternating rows of UV LED lights in thearray of UV LED light assemblies may have different wavelengths. Thesewavelengths may operate in the UV or near UV range, preferably in therange of about 320 nm to about 420 nm, more preferably within the rangeof about 360 nm to about 420 nm.

For example, it was found that for one photocurable formulation that a365 nm wavelength light source yielded a sharp angled dot and that a 395nm wavelength light source yielded a broad angled dot. By using acombination of both 365 nm and 395 nm UV LED light sources, it was foundthat it was possible to get a dot shape that was approximately inbetween the two individual light sources. Thus, in one preferredembodiment, the alternating rows of UV LED light assemblies operate atwavelengths of 365 nm and 395 nm. However, other suitable wavelengths ofUV LED light assemblies and combinations thereof may also be used in thepractice of the instant invention. In addition, the light intensity ofeach of the UV LED light assemblies can also be controlled to provideadditional control and more closely customize the geometriccharacteristics of the relief printing dots.

In the alternative, the array of UV LED light assemblies are positionedin a random pattern of different wavelength UV LED assemblies,including, for example, 365 nm, 395 nm and/or 415 nm as opposed toalternating row of different wavelength UV LED assemblies.

The at least one photocurable layer may comprise any of the knownphotopolymers, monomers, initiators, reactive or non-reactive diluents,fillers, and dyes. In one embodiment, the at least one photocurablelayer comprises a photoinitiator, that has a UV-Vis absorption peak thatis near or in the range of the operating wavelength of the UV LED lightassembly.

In another preferred embodiment, the alternating rows of UV LED lightassemblies are arranged to have different angles of light, measured asthe light impacts the at least one photocurable layer. Thus, it ispossible to use different angles of light and a single wavelength toeffectively control the shape and angle of the printing dots. Forexample, the alternating rows of UV LED light assemblies may comprisecollimated UV LED light assemblies and non-collimated UV LED lightassemblies. In another embodiment, the alternating rows of UV LED lightassemblies may comprise different angles of collimation, whereby thealternating rows impact the at least one photocurable layer at differentangles.

Finally, a combination of different wavelength UV LED lights anddifferent angles of light may also be used to control the shape andangle of the printing dots.

The planarity of the top of a dot can be measured as the radius ofcurvature across the top surface of the dot, r_(t), as shown in FIG. 1.It is noted that a rounded dot surface is not ideal from a printingperspective because the size of the contact patch between the printsurface and the dot varies exponentially with impression force.Therefore, the top of the dot preferably has a planarity where theradius of curvature of the dot top is greater than the thickness of thephotopolymer layer, more preferably twice the thickness of thephotopolymer layer, and most preferably more than three times the totalthickness of the photopolymer layer.

The angle of the dot shoulder is defined as shown in FIG. 2 as the angleθ formed by the dot's top and side. At the extreme, a vertical columnwould have a 90° shoulder angle, but in practice most flexo dots have anangle that is considerably lower, often nearer 45° than 90°.

A dot shoulder angle of >50° is preferred throughout the tonal range. Asused herein, dot shoulder angle means the angle formed by theintersection of a horizontal line tangential to the top of the dot and aline representing the adjacent dot side wall as shown in FIG. 2.

Edge sharpness relates to the presence of a well-defined boundarybetween the planar dot top and the shoulder and it is generallypreferred that the dot edges be sharp and defined, as shown in FIG. 3.These well-defined dot edges better separate the “printing” portion fromthe “support” portion of the dot, allowing for a more consistent contactarea between the dot and the substrate during printing.

Edge sharpness can be defined as the ratio of r_(e), the radius ofcurvature (at the intersection of the shoulder and the top of the dot)to p, the width of the dot's top or printing surface, as shown in FIG.3. For a truly round-tipped dot, it is difficult to define the exactprinting surface because there is not really an edge in the commonlyunderstood sense, and the ratio of r_(e):p can approach 50%. Incontrast, a sharp-edged dot would have a very small value of r_(e), andr_(e):p would approach zero. In practice, an r_(e):p of less than 5% ispreferred, with an r_(e):p of less than 2% being most preferred. FIG. 3depicts a flexo dot and its edge, where p is the distance across the dottop and demonstrates the characterization of edge sharpness, r_(e):p,where r_(e) is the radius of curvature at the intersection of theshoulder and the top of the dot.

In addition, one of the benefits of the invention described herein isthat it is not necessary to perform a bump exposure. During bumpexposure, a low intensity “pre-exposure” dose of actinic radiation isused to sensitize the at least one photocurable layer before the plateis subjected to the higher intensity main dose of actinic radiation. Thebump exposure is applied to the entire plate area and is a short, lowdose exposure of the plate that reduces the concentration of oxygen,which inhibits polymererization of the printing element and aids inpreserving fine features (i.e., highlight dots, fine lines, isolateddots, etc.) on the finished plate. However, the pre-sensitization stepcan also cause shadow tones to fill in, thereby reducing the tonal rangeof the halftones of the image. Thus, the inventors of the presentinvention have found that the use of an array of UV LED light assembliesprovides an acceptable result without the need to perform a bumpexposure.

EXAMPLE I

A study was performed to evaluate photocurable compositions containing avariety of photoinitiators to study how each interacted with twowavelengths (365 nm and 395 nm) light sources. Surprisingly, it wasfound that both the 395 nm and 365 nm light sources produced medium tobroad dots.

Three UV LED assembly units available from UV Process Supply, Inc, andhaving wavelengths of 365 nm, 395 nm and 415 nm, each having anintensity of about 10 to 30 mW were tested. In addition, a unitavailable from Digital Light Labs, Inc., having a wavelength of 365 nmand an intensity of about 175 mW as also tested.

Various formulations of photocurable sheet polymer printing plate blankswere prepared using photoinitiators having a peak around 365 nm. Oneexample of a suitable photoinitiator is2,2-dimethyoxy-1,2-di(phenyl)ethanone available from Ciba SpecialtyChemicals, Inc. under the tradename Irgacure 651.

The UV-Vis spectra of Irgacure 651 at three increasing concentrations isdepicted in FIG. 4.

As may be observed in FIG. 4, Irgacure 651 has absorption peaks at 250and 340 nm. For reference, the UV-Vis spectrum of the 395 nm LEDassembly available the UV Process Supply, Inc. has a very narrow bandwidth, which is typical of UV LED light sources. As such, one mightassume that a 395 nm light source would be outside the usable range forIrgacure I-651. However, it was found that the photopolymer formulationcontaining Irgacure I-651 cured quite well with the 395 nm light source.The band width of the UV Process Supply, Inc. 365 nm LED light source isalso quite narrow.

Two commercially available types of photoinitiators that are advertisedto initiate with higher wavelengths are mono acyl phosphine (MAPO) andbis acyl phosphine (BAPO). MAPO photoinitiators includediphenyl(2,4,6-trimethylbenzoyl)-phosphine oxide, available commerciallyfrom BASF under the tradename Darocur TPO. BAPO photoinitiators includephenyl bis (2,4,6-trimethyl benzoyl) phosphine oxide, availablecommercially from BASF under the tradename Irgacure 819. The UV-Visplots of Darocur TPO and Irgacure 819 are shown in FIGS. 5 and 6. MAPO'sabsorption peaks are at 295 nm, 368 nm, 380 nm and 393 nm. BAPO'sabsorption peaks are at 295 nm and 370 nm. Other photoinitiators thatare advertised to initiate with higher wavelengths include Bis (eta5-2,4-cyclopentadien-1-yl) Bis[2,6-difluoro-3-)1H-pyrrol-1-yl)phenyl]titanium, which is a metallocenethat is commercially available from BASF under the tradename Irgacure784 and which has absorption peaks at 398 and 470.

The main difference between MAPO and BAPO is that BAPO generates tworadicals per molecule when energized, while MAPO only generates oneradical per molecule. In addition to MAPO and BAPO, three additionalcommon UV initiated photoinitiators were examined—Irgacure 651(α,α-dimethoxy-α-phenylacetophenone), Irgacure 184(1-hydroxy-cyclohexyl-phenyl-ketone), and Irgacure 369(2-benzyl-2-(dimethylamino)-1-[4-(4-morpholinyl)-1-propanone).

Table 1 describes various high level and low level photoinitiatorformulations that were used to investigate the use of UV LEDs tocrosslink and cure photosensitive formulations in the preparation ofrelief image printing plates.

TABLE 1 Photosensitive formulations containing various photoinitiators651 651 184 184 369 369 819 819 TPO TPO LO HI LO HI LO HI LO HI LO HIKraton D1102 61.00 61.00 61.00 61.00 61.00 61.00 61.00 61.00 61.00 61.00Ricon 130 26.50 26.50 26.50 26.50 26.50 26.50 26.50 26.50 26.50 26.50HDDA 9.00 9.00 9.00 9.00 9.00 9.00 9.00 9.00 9.00 9.00 Irgacure 651 0.502.00 Irgacure 184 0.50 2.00 Irgacure 369 0.50 1.33 Irgacure 819 0.502.00 TPO 0.50 2.00 BHT 1.20 1.20 1.20 1.20 1.20 1.20 1.20 1.20 1.20 1.20Savinyl Red 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 3BLS

Each photoinitiator was made up at 0.5% and 2.0% levels in thephotosensitive formulation, except that the high level of Irgacure 369and the high level of Irgacure 819 were not nm because they did notdissolve completely. After all of the plates were formulated, their backexposure cure speeds were determined using a standard clam shellexposure unit (fluorescent 365 nm tubes), as set forth in Table 2.

TABLE 2 Back exposure cure speeds of photosensitive formulationscontaining various photoinitiators Time 651 651 369 369 184 184 TPO TPO819 (sec) LO HI LO HI LO HI LO HI LO 0 7 7 7 7 7 7 7 7 7 30 7 7.0 7.016.0 7 7 12.8 14.7 16.2 40 7 1.2 7.0 23.3 7 7 21.0 16.5 23.8 50 7 17.711.8 26.8 7 7 29.2 18.2 32.2 60 7 21.7 17.8 31.0 7 7 29.3 19.0 34.3 70 722.0 7 7 32.3 20.0 36.7 120 7.0 7 7 150 21.8 7 7 180 31.5 7 7 210 39.0 77 240 43.5 7 7

For a 45 G sheet plate, the target floor was 25 to 30 mils. Neitherlevel of Irgacure I-184 yielded any floor after four minutes on the clamshell. Ten full minutes with the high level of Irgacure I-184 was neededto yield a 25 mil floor. However, it was later found that the lack offloor cure by Irgacure 184 was due mainly to the PET that was used. Whena low UV absorbing PET was used, the Irgacure 184 easily cured thefloor.

It was interesting that some of the photoinitiators actually built afloor faster than Irgacure I-651, which is commonly used in printingplate formulations. It was noted that the high level Darocur TPO formeda floor slower than the lower level Darocur TPO, which may be due to theabsorption via the cleaved Darocur TPO molecule. The plates were made onDTF628 polyethylene terephthalate (PET) at 045 G thick. The DTF628 PETabsorbs a lot of light at 365 nm, thus it is believed that the use of adifferent PET may yield faster floor building results.

Thereafter, digitally ablated plates with pre-cured floors were placedunder the different UV LED light sources on a UV track system, which wasdesigned to be very stable at very slow operating speeds. A graphic ofline speeds for specific percent inputs was mapped out and is shown inFIGS. 7 and 8.

As may be observed in the plots, the speed is essentially linear from20% input to 80% input. The fastest forward direction is about 4.2 fpm.The top speed is about 7 fpm and occurs on the return (reverse)direction, but is not adjustable. For purposes of this study, a linespeed of 0.13 fpm (10% input) was chosen. The output was not limited andthe height from the light source to the plate surface was set to 0.5inch.

FIG. 9 depicts the floor building profile utilizing the high level TPOformulation in combination with the UV Process 395 nm LED light source.The UV track system was run at specific settings and the plates wereprocessed (i.e., developed) in solvent.

FIG. 10 depicts SEM images of dots from the combination of the DigitalLight 365 nm unit with each of the photoinitiator formulations tested.

As can be seen, several of the formulations tested exhibited excellentdot shape under a 365 nm light source. While the low level IrgacureI-189 formulation started to form flat dots, the Irgacure I-651, DarocurTPO and Irgacure I-369 formulations exhibited true flat topped dots. Thelow level of each photoinitiator formulation evaluated exhibited thestandard rounded digital dot formation. While the Irgacure I-651 dotswere like telephone poles, it appeared that the Darocur TPO and IrgacureI-369 formulations would print well.

FIG. 11 depicts the SEM images of dots for different formulations curedwith the UV Process 395 nm light unit.

The Irgacure I-184 and Irgacure I-651 formulations are not shown becausethey were incapable of holding any dots. The formulations holding themost promise for dot formulation using a 395 nm light source containedDarocur TPO and Irgacure I-369 as the photoinitiator.

However, only the TPO formulation was observed to form acceptable dotswith the UV Process 415 nm LED assembly as seen in FIG. 12.

The best dot formation was at 365 nm with the Irgacure I-651 and DarocurTPO photoinitiators, and with the Darocur TPO photoinitiator at 395 nm.Therefore, it was decided to attempt to combine two wavelengths in aserial exposure sequence. The high level Darocur TPO formulation wascured first with one wavelength and then immediately after by the otherwavelength. The order of the light sources was then switched and runagain with the high level Darocur TPO.

FIG. 15 shows the results of these light combinations with Darocur TPO.The individual wavelength runs are shown as well for comparison as is aclam shell exposure. The results show that the dots were an average inwidth between the two individual exposures. Thus it can be seen that aspecific design of a UV LED assembly array can be optimized for aspecific dot formation.

The Irgacure I-651 formulation was not run because there was little dotformation seen at the 395 nm wavelength, although it is believed that itwould be possible to use a higher ratio of 365 nm versus the 395 nmwavelength to adjust the dot shape.

It was surprisingly discovered that the best dot formation occurred withthe Darocur TPO and a 365 nm wavelength LED light source. Anothersurprise was how well the Irgacure I-369 photoinitiator performed withthe 365 nm LED light source.

The results indicated that the use of a mixture of wavelengths mayminimize the effect. That is, instead of using two different lightsources in succession, a light source can be constructed so that bulbsof different wavelengths are dispersed throughout the array.

For example, the UV LED light assembly may comprise an array arranged asfour rows of bulbs. The first and third rows of the array may be 365 nm,while the second and fourth rows of the array may be 395 nm. Eachwavelength can then be on a separate circuit which would have theability to modulate the intensity separately, allowing the user tocustomize the dot formation.

What is claimed is:
 1. A method of tailoring the shape of a plurality ofrelief printing dots created in a photosensitive printing blank during aplatemaking process, said photosensitive printing blank comprising atleast one photocurable layer disposed on a backing layer, the methodcomprising the steps of: a) selectively exposing the at least onephotocurable layer to a source of actinic radiation to selectivelycrosslink and cure the at least one photocurable layer; and b)developing the exposed at least one photocurable layer of photosensitiveprinting blank to reveal the relief image therein, said relief imagecomprising the plurality of relief printing dots; wherein the source ofactinic radiation comprises an array of UV LED light assemblies and theuse of the array of UV LED light assemblies produces relief printingdots having at least one geometric characteristic selected from thegroup consisting of a desired planarity of a top surface of the reliefprinting dots, a desired shoulder angle of the relief printing dots anda desired edge sharpness of the relief printing dots.
 2. The methodaccording to claim 1, wherein the array of UV LED light assembliescomprises at least four rows.
 3. The method according to claim 2,wherein alternate rows of UV LED lights in the array of UV LED lightassemblies have different peak wavelengths.
 4. The method according toclaim 3, wherein the alternate rows of UV LED lights in the array of UVLED light assemblies operate at wavelengths in the range of 360 nm to420 nm.
 5. The method according to claim 3, wherein the alternate rowsof UV LED light assemblies operate at wavelengths of 365 nm and 395 nm.6. The method according to claim 1, wherein the at least onephotocurable layer comprises a photoinitiator, wherein saidphotoinitiator has a UV-Vis absorption peak in the range of theoperating wavelength of the UV LED light assembly.
 7. The methodaccording to claim 1, wherein alternate rows of UV LED light assembliesare arranged to have different angles of light.
 8. The method accordingto claim 7, wherein the alternate rows of UV LED lights comprisecollimated UV LED light assemblies and non-collimated UV LED lightassemblies.
 9. The method according to claim 1, wherein the angle of therelief printing dot shoulder is greater than about 50°.
 10. The methodaccording to claim 1, wherein the edge sharpness of the relief printingdots, defined as the ratio of the radius of curvature r_(e) at theintersection of a shoulder and the top of the relief printing dot to thewidth of the dot's top printing surface p is less than about 5%.
 11. Themethod according to claim 11, wherein the ratio of r_(e):p is less thanabout 2%.
 12. The method according to claim 1, wherein a bump exposureis not performed.
 13. A method according to claim 1, wherein the arrayof UV LED light assemblies comprises at least two LED light assemblieswhich have peak wavelengths that are different from each other.
 14. Amethod according to claim 1 wherein the relief printing dots comprise aplanarity that is greater than the thickness of the at least onephotocurable layer.